The gamma chain gene of the IL-2 receptor (IL2RG) has been identified as responsible for X-linked severe combined immunodeficiency (SCID). This is the most common form of human SCID. The only available lifesaving treatment at the present time is bone marrow transplantation, but drawbacks of this procedure include the lack of an HLA matched donor in over 2/3 of cases and poor post-transplant B cell function, primarily due to non-engraftment of B cells. The previously proven negative selection against cells expressing a mutant IL2RG, resulting in skewed X chromosome inactivation in female carriers, makes X-linked SCID a promising candidate for therapy with retrovirally transduced autologous repopulating stem cells. This strategy is being pursued first by comparing different retrovirus constructs for titer, gamma chain expression in transduced cell lines, correction of the functional defect in EBV-B cell lines from SCID-affected patients with defined IL2RG defects, and monitoring the development of cord blood stem cells transduced with IL2RG retroviruses in SCID mice. These experiments have shown significant cell surface expression of the gamma chain protein by immunofluorescence in previously deficient cell lines from SCID patients. The availability of a large panel of patient cell lines will permit evaluation of potential dominant negative mutations. SCID patients prenatally diagnosed with defined IL2RG mutations have had their cord blood collected at birth for experimental transduction and evaluation of lymphocytogenesis. Canine spontaneous gamma chain mutations resulting in SCID dogs are being studied in collaboration with Paula Henthorn, at the U. of PA School of Vet. Med. A marking experiment is in progress in which two retroviral constructs were introduced into bone marrow of an unaffected dog pretreated with cytokines. The marrow was infused into a MHC-matched irradiated recipient, who at 8 weeks demonstrates substantial blood and bone marrow expression of human gamma chain. Follow-up experiments in SCID dogs are planned.